A lift truck is an industrial vehicle that is utilized to lift and transport loads between locations. A lift truck generally includes a set of rails, or fork that is positioned under a pallet which is to be moved. The fork has an adjustable height that, once positioned under a pallet, is raised by a jack or other means to lift the pallet off the ground. Once the pallet has been lifted by the fork, the lift truck may be used to move the pallet to a new location.
One common application for lift trucks is picking up and transporting loads in a warehouse environment. Lift trucks used in a warehouse environment are often motorized to facilitate moving large loads and to increase the speed with which loads may be transported. Motorized lift trucks typically allow the operator to ride in the truck and drive the truck from one location to the next to increase the speed and efficiency with which pallets are moved.
Lift trucks are often used by workers in a warehouse environment, such as a distribution center, to assemble or pick an order from available inventory. To facilitate the ability to locate a desired inventory item, warehouses are typically organized into numbered aisles having storage racks on either side of the aisle. The storage racks are further subdivided into adjacent numbered slots, so that there are numerous storage areas within the warehouse with each storage area being uniquely identifiable by its aisle and slot number. A warehouse management system running on a centralized computer is typically used to keep track of items by their location in the warehouse, so that any item in stock may be located quickly by an aisle and slot number. To assemble an order, the lift truck operator, or order selector is provided with a list of items generated by the warehouse management system that includes the location of each item so that the items may be placed together on a pallet.
The item list may be in the form of a printout, but more typically will be conveyed to the order selector using a voice-directed warehousing system. Such voice-directed systems typically use speech synthesis and recognition technology to maintain a speech dialog with the order selector to facilitate assembling an order. For example, the order selector may wear a headset and/or portable, mobile computer that provides two-way speech communications. A larger centralized computer system maintains the item lists for various orders, and communicates with the mobile computers and order selector. The voice-directed system, through a voice application or task loaded on the mobile computer, directs the order selector to an item that is to be loaded onto the pallet—or picked—using synthesized speech commands. The speech commands identify the item to be picked and its location by aisle number and slot number so the order selector knows where to move the lift truck, and pick the item. Once the lift truck has been moved to the correct aisle and slot number, the order selector exits the lift truck and picks the item. The order selector then indicates to the voice-directed system that the item has been picked by speaking into the headset. The voice-directed system then directs the order selector to the next item to be picked and the selector drives the truck to the appropriate aisle and slot, with the process continuing until the order is complete. Voice-directed systems thus allow workers to keep their hands and eyes free to pick items and load them onto the pallet, as well as operate the lift truck, while maintaining communication with the voice-directed warehousing system.
When assembling orders, order selectors are typically required to visit several locations within the warehouse. To minimize the time required to assemble an order, the picking order is typically organized so that items which are located in close proximity or in the same aisle will be picked sequentially. In this way, the order selector may be directed to items in a logical sequence, which minimizes the distance between picks and the drive time, and thereby increases efficiency. As a result, multiple items will often be sequentially picked from slots in the same aisle, requiring the order selector to repeatedly exit and enter the lift truck within a very short period of time along the aisle. The order selector thus spends unproductive time and energy stepping onto the truck to move it a short distance down an aisle before stepping back off the truck to pick the next item. Thus, while picking items from a single aisle, it would be desirable for the order selector to be able to walk alongside the lift truck and pick the items while the lift truck continues to move in the direction of the next item, rather than requiring them to continually jump on the truck, drive it a short distance, and jump off.
Previous attempts to allow lift trucks to move autonomously through a warehouse have used very expensive and complicated navigation systems that rely on location indicia fixed to the ceiling or floor of the facility, such as signal reflectors or coded indicia. However, these systems require that the coordinates of the indicia be known precisely with respect to the location of objects, such as storage racks, within the warehouse. Furthermore, the entire facility has to be mapped with the indicia and/or sensors to provide proper navigation. This requirement adds significant implementation costs. Moreover, any changes to the configuration of aisles within the warehouse will require that the system be reprogrammed and/or location indicia moved so that the system knows the new warehouse configuration. Indicia fixed to the ceiling are typically positioned far enough from the lift truck so that accurate positioning of the truck is difficult, while indicia fixed to the floor must withstand the wear and tear caused by vehicle wheels. A further challenge is that the distance the truck has moved is typically determined based on wheel rotations, and the diameter of a lift truck tire may vary by as much as 25% over its useful life. Therefore, with such a variation, it is difficult to determine the speed and distance travelled for a lift truck. Thus, the current solutions to determining the location of lift trucks in a warehouse environment have proven to be more expensive and less reliable than desired.
Therefore, there is a need for improved methods and systems for navigating lift trucks within a warehouse. Further, there is a need to allow lift trucks to accurately and autonomously move around a warehouse based on the location of the lift truck and the known location of the inventory items to be picked so that the time and effort expended by the order selector is reduced. Furthermore, there is a need for such a system that is minimally impacted by any changes to the configuration of the aisles within the warehouse.